Isotachophoresis (ITP) separates charged molecules (ions) in a sample based on their different effective mobility. Effective mobility is the proportionality constant between observable drift velocity of an ion and applied electric field. It is a function of ion shape, size, and the degree of ionization.
ITP uses a leading electrolyte (LE) which contains a relatively high mobility ion, and a trailing electrolyte (TE) with a relatively low mobility ion. The TE and LE ions are chosen to have effective mobilities respectively lower and higher than target analyte ions of interest. That is, the effective mobility of analyte ions is higher than that of the TE and lower than that of the LE. These target analytes have the same sign of charge as the LE and TE ions (i.e., a co-ion). An applied electric field causes LE ions to move away from TE ions and TE ions to trail behind. A moving interface forms between the adjacent and contiguous TE and LE zones. This creates a region of electric field gradient (typically from the low electric field of the LE to the high electric field of the TE). Analyte ions in the TE overtake TE ions but cannot overtake LE ions and accumulate (“focus”) at the interface between TE and LE. Alternately, target ions in the LE are overtaken by the LE ions; and also accumulate at interface.